1. Field of the Invention
This invention relates generally to filter elements and in particular to compact hybrid multi-pole filter elements that are useful in the design of radio frequency filters.
2. Description of Related Art
Conventional radio frequency filters have been constructed by a variety of different elements. Some of these elements include lumped reactive elements, distributed reactive stubs, and impedance transformers. Each of these conventional elements and techniques has its advantages and disadvantages.
For example, the lumped elements usually provide the smallest foot print. However, lumped elements also have the highest insertion loss and are suitable only for low power. FIG. 1 is an example of a lumped element model of a low-pass filter (LPF). The reactance of the inductors is XL=jxcfx89l and the capacitor reactance is XC=1/jxcfx89C. These equations are well known to those skilled in the art. At low frequencies XL=0xcexa9 and XC=∞xcexa9, these characteristics allow low frequency signals to pass through the inductors between P1 and P2. Alternatively, at high frequencies XL=∞xcexa9 and XC=0xcexa9, these characteristics prevent high frequency signals from passing through the circuit between P1 and P2. FIG. 2 is a graph of the reflection coefficient (xcex93) of a capacitor and inductor.
Alternatively, reactive stubs have low insertion loss, but can be physically large. Also, reactive stubs tend to have a narrow bandwidth, which can make them unsuitable for wideband applications. FIG. 3 is an example of a LPF using xcex/4 wave open-end stubs. A LPF composed of distributed elements is possible by having opened-ended stubs (i.e., reactive stub) of xcex/4 wave length at the frequency that is rejected. This causes the stub to appear as an open circuit above and below a rejection frequency. At the rejection frequency, the stub appears as a short circuit thereby allowing no signal flow. The stubs are also spaced 90xc2x0 electrically apart at the pass band of a fundamental frequency for matching purposes. This filter element produces nulls that are deep. However, the tradeoff is a small bandwidth of 2%, as shown in FIG. 4. Therefore, numerous stubs are required to obtain a wide bandwidth. Although a very good response can be obtained, the numerous stubs produce an extremely long filter element. Thus, these elements are not suitable for compact circuits.
FIG. 5 is an example of a LPF using impedance transformers. A distributed impedance transformer LPF typically has a wide bandwidth. However, the nulls of the filter are very small. The filter design comprises transforming from a high to low impedance until the desired bandwidth is reached. Because of the large bandwidth of the transformers, few transformers are needed. In addition to the very small nulls produced by this design, the roll off of the circuit, as shown in FIG. 6, is so gradual that it can interfere with the pass band. Those skilled in the art will appreciate that the low nulls and gradual roll off characteristics of the transformer design make it unsuitable for many applications.
Accordingly, it is an object of the present invention to provide an improvement of filter elements and filter designs.
It is yet another object of the invention to provide a hybrid filter element that has advantageous features of both opened stub and impedance transformer designs.
The foregoing and other objects are achieved by a hybrid filter element comprising an impedance transformer and a phase cancellation loop having a first portion and a second portion. The first and second portions are designed to provide a phase difference between the two portions of about 180xc2x0 at a mid-band frequency. The first portion can form part of the impedance transformer. Further, the first and second portions can be designed to have either an equal power split or an unequal power split between the portions.
Further scope of the applicability of the present invention will become apparent from the detailed description provided hereinafter. However, it should be understood that the detailed description and specific embodiments, while disclosing the preferred embodiments of the invention, are provided by way of illustration only inasmuch as various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description which follows.